1. Field of the Invention
The present invention relates to a volume hologram layer used for a volume hologram transfer foil and the like and to a volume hologram transfer foil using the volume hologram layer.
2. Description of the Related Art
Holograms are those obtained by allowing two lights (object light and reference light) having the same wavelength to interfere with each other to record the wavefront of object light on a photosensitive material as an interference fringe. When light of the same condition as the original object light is applied to this hologram, a diffraction phenomenon caused by interference light arises, making it possible to reproduce the same wavefront as the original object light. These holograms are classified into several categories (e.g., a surface relief type hologram and volume hologram) by the recording forms of interference fringes caused by the interference of laser light or highly coherent light.
Here, the aforementioned holograms are used a lot in security applications utilizing such characteristics that it is difficult to reproduce the same design. In this application, a surface relief type hologram is generally used in which fine irregularities are formed on the surface of a hologram forming layer to thereby record an interference fringe. However, the recent spread of hologram technologies and advancement in forging techniques have made it possible to copy and reproduce the surface relief type hologram and it has became difficult to use the surface relief type hologram in security applications.
On the other hand, the volume hologram is produced by recording an interference fringe caused by the interference of light, as a fringe differing in refractive index, three dimensionally in the direction of the thickness of a photosensitive material. This volume hologram differs in manufacturing method and also quite different in visual effect from the surface relief type hologram, which is widely used at present, it is easy to determine the authenticity. Also, the volume hologram requires expensive manufacturing facilities and a high grade design techniques to copy a design, it is therefore very difficult to produce a hologram with the intention of forging. For this, the volume holograms in use in security applications are demanded.
Here, in order to make a hologram adhere to an adherend, a transfer foil or the like comprising a base material film, a hologram layer and a heat sensitive adhesive layer is usually used. In this case, the heat sensitive adhesive layer in the hologram transfer foil is brought into contact with the adherend and heat transfer is carried out from the base material film side. At this time, in order to form the hologram layer into the shape to be intended, it is necessary that the cutting property of the hologram layer, namely, foil cutting property is good that the hologram is surely imparted to the adherend. However, because the volume hologram has the features that an interference fringe is recorded in the forming layer, it is necessary to thicken the volume hologram layer to obtain a clear image, resulting in inferior foil cutting property. Also, for mass production of the volume hologram transfer foils, it is necessary to use a photopolymer which can be treated by a dry process. However, because nonreactive plasticizers are usually used to improve refractive index modulation ability, a produced volume hologram is softened, which is accompanied by inferior foil cutting property.
As such a volume hologram, for example, a photosensitive composition for recording a volume hologram with high film strength (see Japanese Patent No. 2873126) and a label for forming a volume hologram laminated body to which a fine particle is added (see Japanese Patent Application Laid-Open (JP-A) No. 10-97173) have been proposed. Also, a hologram transfer foil whose breaking strain is defined has been proposed (see JP-A No. 2000-272295). However, any of these methods have not yet reached a hologram layer which has a clear image, excellent transfer ability and can be mass produced.